1. Field of the Invention
This invention relates to a method for producing fine silver-palladium powder by chemical reduction, and more particularly to a method for producing fine silver-palladium alloy powder which can be carried out in a continuous mode and in which the specific surface area of the silver-palladium alloy powder can be controlled.
2. Description of the Related Art
Silver-palladium powder is formulated with glass powders and resins to form pastes that are used in thick film conductive circuits. The surface characteristics of silver-palladium powders vary depending upon their specific surface areas, the particle sizes, the particle size distributions and the tap densities. Some well known methods of producing silver-palladium powder include (1) blending silver and palladium powders under mechanical stirring, (2) an electrochemical reduction method, (3) a spray-pyrolysis method and (4) a chemical coprecipitation method. There are some problems encountered in the blending method. For example, it takes a very long time to obtain a powder blend, and a homogeneously blended powder is hardly achieved. Furthermore, the blended powder undergoes abnormal expansion and shrinkage during heat treatment (200.degree.-500.degree. C.). This tendency becomes more severe as the thickness of the conductive film becomes thinner. When the electrochemical reduction method is used, the resulting silver-palldium powder is generally nonhomogeneous. In addition, a lot of time and energy is consumed, thereby increasing the production cost. Thus, the electrochemical reduction method is not suitable for commercial use.
In J. Mater Sci. 26 (1991) 2477-2482, Nagashima et al. suggested that silver-palladium alloy powder can be produced by spray-pyrolysis. Although the silver-palladium alloy powder so produced has a sintering characteristic better than that of the mechanically mixed silver-palladium powder, it has a relatively large particle diameter (0.1-10 .mu.m), thereby limiting its application.
Accordingly, presently available commercial silver-palladium powders are generally produced by chemical coprecipitation. Conventional coprecipitation processes for the production of silver-palladium powders include the steps of: mixing two solutions prepared by dissolving silver and palladium separately in two nitric acid solutions; and adding a reducing agent, such as hydrazine, formaldehyde or hypophosphorous acid, into the mixed solution to reduce and coprecipitate silver and palladium ions as silver-palladium alloy particles. Filtering, washing and drying follow after coprecipitation so as to obtain the silver-palladium alloy powder. Such a process has been disclosed in U.S. Pat. Nos. 3,390,981 and 4,776,883.
U.S. Pat. No. 4,776,883 to Hayashi et al. further suggests a subsequent heat-treatment step after coprecipitation to obtain fine silver-palladium powder having characteristics satisfying the requirements of ceramic capacitors. In particular, the obtained powder is subjected to a subsequent heat treatment at a temperature of 100.degree.-500.degree. C. under an inert atmosphere or vacuum. The additional heat treatment step complicates the process and increases the production cost.
U.S. Pat. No. 5,292,359 teaches a process in which water is added as a diluent to the mixture of silver and palladium nitrate solutions in order to adjust the concentration and the pH of the solutions and in which the resulting solutions are mixed with a mixture containing a reducing agent and a surfactant for reduction and coprecipitation. This reduction and coprecipitation process is a batch-type process. Since reduction and surface treatment take place at the same time in the process, the production procedure is simplified and is suitable for mass production. In addition, the resulting powder is fine and suitable for forming electronic conductive films. Nevertheless, the process involves the risks that a mixed type silver and palladium powder is produced rather than an alloy type silver-palladium alloy powder and that the product quality is unstable and varies from batch to batch. Accordingly, the process cannot provide a high quality product with a predetermined specific surface area and particle size.
Until now, batch type processes have been suggested for the production of silver-palladium alloy powders and such processes are generally difficult for the production of powders having controlled or predetermined surface characteristics such as specific surface area, particle size distribution, and tap density. As per the requirements of the thick film electronic industries, silver-palladium powders should be an alloy and the surface characteristics of the alloy powders should be matchable with glass powders and resins to ensure the formation of high quality conductive thick films. Further developments in this aspect are thus desirable.